For the process gases in a biogas plant, first of all, the following are defined:                Crude biogas: process gas which is withdrawn from the fermenter.        Processed crude biogas: crude biogas which has already passed through a biogas desulfurization and gas drying, but not a CO2-separation stage.        Biogas: process gas which, after the CO2-separation, predominantly comprises biomethane. The biogas can be fed into the natural gas grid after corresponding conditioning and compression.        
It is known to process crude biogas from fermentative production and obtain biomethane as biogas. The crude biogas, depending on the composition of the substrate of the biogas plant, contains methane concentrations of approximately 40-75% by volume and carbon dioxide in a concentration range from approximately 25 to 55% by volume. The crude biogas is saturated with water vapor at the fermentation temperature and can have additional minor components. The processing of the crude biogas therefore comprises for the most part the following three method steps, biogas desulfurization, gas drying and CO2-separation. The biogas can, after corresponding conditioning and compression, subsequently be fed into the natural gas grid.
For CO2-separation, adsorptive separation methods are known, using zeolites (WO09/58726 or WO2008/072215) or carbon molecular sieves, and also absorptive separation methods such as physical scrubbing by means of water or Genosorb® or chemical scrubbing using MEA, DEA or MDEA. For said methods, depending on usage of the adsorber or absorber, either the CO2 is separated off from the methane at high pressure of 4-7 bar, and regeneration of the adsorber or absorber is connected downstream at a low pressure (pressure-swing method), or the CO2 is bound at low temperature and the adsorber or absorber is regenerated at high temperature (temperature-swing method). Regeneration of adsorbed CO2 by way of moderate temperatures or application of a vacuum is disclosed by U.S. Pat. No. 5,797,979. Said pressure- and temperature-swing methods are energy-intensive.